Soluble cyclic GMP phosphodiesterase activity is present in culture fibroblasts. Activity is stimulated as much as 3-fold by caluium and calmodulin. The amount of cyclic GMP phosphodiesterase activity detected is dependent upon the methods used to harvest the cells and to prepare subcellular fractions. Homogenization of cells substantially reduces cyclic GMP phosphodiesterase activity and calmodulin-sensitive activity. Harvesting cells by scraping and centrifugation at 10,000 x g or 100,000 x g reproducibly yields a supernatant fraction which contains 90% of the total cyclic GMP phosphodiesterase activity and calmodulin-sensitive activity detected before centrifugation.